1. Field of the Invention
The present invention relates to an image forming apparatus which conducts light from a light source to an optical shutter array constructed of PLZT or similar material possessing electro-optical effects, applies an actuation voltage to appropriate optical shutters based on image signals, thereby allowing light to be transmitted therethrough so as to form an electrostatic latent image on a photosensitive member, and more specifically relates to an image forming apparatus which applies a suitable actuation voltage to optical shutters in accordance with the type of light generated by the light source and the spectral sensitivity of the photosensitive.
2. Description of Related Arts
Optical shutters constructed of PLZT or like material possessing electro-optical effects, in principle, pass light generated by a light source 1 through a polarizer 2 which is the first polarizing plate to optical shutters 3, as shown in FIG. 1, such that when an actuation voltage from a power source is applied to electrodes 4 provided on both sides of said optical shutters 3, the light entering said optical shutters 3 is polarized and transmitted through an analyzer 6 which is a second polarizing plate.
The use of such optical shutter arrays having the aforesaid characteristics as writing devices in image forming apparatus has heretofore been investigated. Conventionally, light from a light source passes through a polarizer and is transmitted to an optical shutter array, whereupon an actuation voltage is applied to the appropriate optical shutters based on image signals, the transmitted light is polarized, passes through an analyzer and thereafter said transmitted light forms an electrostatic latent image on a photosensitive member.
When an electrostatic latent image is formed on a photosensitive member, therefore, it is desirable that the light transmitted from the optical shutter be of high intensity and have characteristics which coincide with the spectral sensitivity of the photosensitive member.
Optical shutter characteristics are shown below in Equation 1, wherein the intensity I of the light transmitted from the optical shutter is expressed relative to the intensity Io of the light entering said optical shutter after passing through a polarizer. EQU I/I.sub.o =Sin.sup.2 (.pi.LN.sup.3 RE.sup.2 /2.lambda.) [I]
[In the equation, L is the optical path length, n is the refractive index, R is the Kerr constant, E is the electric field strength, and .lambda. is the light wavelength.]
As shown in Eq. 1, the intensity I of the light transmitted from the optical shutter changes according to the electric field strength E induced by the actuation voltage and the light wavelength .lambda..
Therefore, to increase the intensity I of the light transmitted from the optical shutter, the actuation voltage must be altered in accordance with the wavelength .lambda. of the light produced by the light source.
When a monochromatic light source is used which produces light of a single wavelength, an optimum actuation voltage to apply to the optical shutters can be readily determined. When a white light light source such as a halogen or xenon lamp is used, however, the light generating from said light source has a wide range of wavelengths, so that the quantity of light at each wavelength must be considered to determined the optimum actuation voltage.
In addition, even though the actuation voltage is set to maximize the intensity I of the transmitted light, the characteristics of said transmitted light may not coincide with the spectral sensitivity of the photosensitive member so that the optimum actuation voltage is not set for the exposure of the photosensitive member and the formation of an electrostatic latent image thereon. Therefore, the characteristics of the entering light and transmitted light as well as the spectral sensitivity of the photosensitive member must be considered.
Thus, the application of an optimum actuation voltage to the optical shutters so as to expose a photosensitive member and form an electrostatic latent image thereon makes the operation of the image forming apparatus extremely complex.
The use of halogen and xenon lamps as representative examples of white light sources are described hereinafter.
First, the spectral (energy) distribution in a specific wavelength range was investigated for light generated by halogen and xenon lamps as lights sources, and the results are shown in FIG. 2. In FIG. 2, item 1 shows the spectral energy distribution for a halogen lamp at a distribution temperature of 3,400 mK, and item 2 shows the distribution for a xenon lamp at peak energy (100%) within the visible light range.
FIG. 3, on the other hand, shows the spectral sensitivities of three representative types of photosensitive members.
Comparison of the spectral energy distributions of light generated from halogen and xenon lamps shown in FIG. 2, and the spectral sensitivity characteristics of photosensitive members A, B and C shown in FIG. 3 clearly shows that the aforesaid spectral distributions and spectral sensitivity characteristics did not necessarily coincide.
Thus, setting the actuation voltage according to Eq. 1 based on the wavelength at which the spectral energy distribution is maximized for the aforesaid light sources does not necessarily produce the optimum actuation voltage for exposure of the photosensitive member.
Halogen and xenon lamp light wavelength .lambda. was set at 650 nm, 550 nm and 450 nm and suitable actuation voltages were determined for each using Eq. 1. The thus derived actuation voltages were applied to the optical shutters and spectral energy distributions were determined for the light transmitted from said optical shutters within a fixed wavelength range.
The results of the investigation of the halogen lamp are shown in FIG. 4 and the xenon lamp in FIG. 5. In the figures, 1 is the 650 nm setting, 2 is the 550 nm setting and 3 is the 450 nm setting.
For example, when considering exposure of photosensitive member B of FIG. 3 using a xenon lamp as the light source, it is apparent that spectral energy distribution 3 of FIG. 5 more closely coincides with the spectral sensitivity of the photosensitive member than does spectral distribution 1. It is therefore desirable that, in the case, light wave .lambda. be set at 650 nm in determining the actuation voltage to be applied to the optical shutters. When another photosensitive member is used, however, the application to the optical shutters of the actuation voltage set for photosensitive member B may not necessarily be the optimum actuation voltage for exposure of the photosensitive member because the spectral characteristics of said photosensitive member differ from those of the aforesaid photosensitive member B. Conversely, when the light source is changed, the application of the actuation voltage determined as described above may be undesirable for the exposure of the photosensitive member even if photosensitive member B is again used.
In the aforesaid image forming apparatus, therefore, the spectral energy distribution of the light source and the spectral sensitivity characteristics of the photosensitive member must be checked each time an electrostatic latent image is formed on the photosensitive member in order to apply the optimum actuation voltage to the optical shutters. The aforesaid process makes the operation of the image forming apparatus under optimum conditions extremely complex.